The present application relates to an integrated ground fault detector.
Integrated circuits have become increasingly common in motor drive applications. Many features that were previously realized using discrete components or multi-board systems in the past are now implemented using small and reliable integrated circuits (IC's). This allows users to save area on a board and provides better performance when compared to that of discrete solutions.
In one application, an inverter for use in a triphase motor, the prevention of motor and inverter faults is critical. Some types of inverter faults may be detected using a shunt resistor on the DC bus of the inverter. This shunt is commonly used to sense a current that is used by a controller to reconstruct the phase currents of the motor to control torque and speed thereof. However, this shunt may also be used to provide information about certain faults.
FIG. 1 illustrates an inverter circuit 1 in which a shunt resistor shunt1 is provided on the negative rail DC−bus of the DC bus between the rectifier bridge 10 and the inverter 12. While FIG. 1 illustrates a rectifier bridge 10 for providing the DC bus voltage, any suitable voltage source may be used. The shunt resistor shunt1 may be used to detect a short circuit in an inverter leg, that is, where the switching of the switches in the inverter creates a short circuit and/or a short between the phases of the motor, which is represented by switch s in FIG. 1. The dashed lines in FIG. 1 represent short circuit current paths. Another conventional way to detect faults is to sense the voltage drop on the power switches of the inverter. This is usually implemented inside the gate driver IC and is generally referred to as desaturation protection. However, this approach is difficult to implement since the power switches always have at least one terminal moving between the positive and negative two bus voltages.
Some faults can only be detected using a shunt resistor on the positive DC bus rail DC+bus. Thus, shunt1 of FIG. 1 is incapable of detecting these faults. FIG. 2 illustrates the inverter circuit 1 of FIG. 1 with a second shunt resistor shunt2 added to the positive DC rail DC+bus. The switch a in FIG. 2 represents a short between the positive DC bus DC+bus and the motor frame. The switch b represents a short between a motor phase and the motor frame. Since the motor frame is generally grounded, these shorts would avoid the negative DC bus rail DC−bus, and thus, would not be detectable by the shunt resistor shunt1, for example, in FIG. 1. Thus, at least two shunt resistors would be needed in order to provide the bare minimum of short circuit fault protection required for the inverter circuit 1
Other solutions to fault detection utilizing discrete components and use a large number of components. As a result, these solutions usually have low reliability and take up a substantial amount of area.
Thus, it would be beneficial to provide a fault detection circuit that avoids the problems mentioned above.